1. Technical Field
The present invention relates to a method of determining the adequacy of acquisition of a satellite signal and the like.
2. Related Art
As one type of positioning system using a positioning signal, GPSs (Global Positioning Systems) are widely known and are used in receiver devices built in cellular phones, car navigation equipment, and the like. In the GPS, a position calculating process is performed in which the coordinates of the position and the clock error of the receiver device are acquired based on information of the positions of a plurality of GPS satellites, pseudo distance from each GPS satellite to the receiver device, and the like.
A GPS satellite signal that is transmitted from a GPS satellite is modulated with a spread code that is called a C/A (Coarse and Acquisition) code and is different for each GPS satellite. The C/A code is a pseudo random noise code, in which a code length of a 1023 chip is set as 1 PN frame, having a repetition period of 1 ms. In order to acquire a GPS satellite signal from among weak reception signals, the receiver device calculates the correlation between a reception signal and a replica C/A code that is a pseudo C/A code generated inside the receiver device and acquires a GPS satellite signal based on a resultant correlation value that is acquired.
Since the C/A code is a so-called gold code, a high correlation value is not acquired unless a correlation with a replica C/A code of a correct GPS satellite signal is calculated. However, the overall magnitude of the acquired correlation value may vary depending on the reception status of a GPS satellite signal. Accordingly, for example, in a case where the signal strength of the reception signal is high or the like, even when a correlation with an incorrect satellite replica C/A code is calculated, a correlation value that is equal to or larger than a predetermined value is acquired, whereby there is a case where the correlation is determined to be acquired. Such a case is a cross correlation.
In the description here, the correlation of a case where a satellite assumed by a GPS receiver device matches an actual satellite is defined as a “correct correlation”. In contrast to this, the correlation of a case where a satellite assumed by the GPS receiver device is different from an actual satellite is defined as a “cross correlation”. The occurrence of a cross correlation indicates that a satellite signal is acquired from a satellite other than another satellite that is assumed by the GPS receiver device. This indicates that the acquisition of a GPS satellite signal is not appropriately performed. When the cross correlation occurs, there is a problem in that the accuracy of calculation of a position decreases. Thus, various techniques used for determining a cross correlation are proposed (for example, JP-A-2003-84055).
According to the technique disclosed in JP-A-2003-84055, a cross correlation is determined by performing a threshold value determination process for a correlation value acquired by the correlation calculation in a stepped manner. In the technique disclosed in JP-A-2003-84055, in order to realize the determination of a cross correlation, it is necessary to appropriately set stepwise threshold values. Meanwhile the reception status such as the signal strength or the reception environment at the time of receiving a GPS satellite signal by using the receiver device changes every time. Accordingly, in a case where a cross correlation is determined based on a uniform determination criterion, there is a problem in that the accuracy of the determination is not assured. Being unable to correctly detect a cross correlation is being unable to correctly determine the adequacy of the acquisition of a satellite signal.